Studies on the biosynthesis of riboflavin will focus on the mode of formation of the two 4-carbon units making up the o-xylene moiety of this molecule from carbohydrate precursors. This problem will be studied in Ashbya gossypii using 13C-NMR spectroscopy and using cell-free systems. Mechanistic aspects of the transformation of a carbohydrate precursor into the 4-carbon units of dimethylribityllumazine and riboflavin will be examined with the chiral methyl group methodology. Again by stable isotope and NMR techniques the mode of conversion of riboflavin into the dimethylbenzimidazole moiety of vitamin B12 will be studied. Finally, the mode of biosynthesis of 8-hydroxy-7, 8-didemethyl-5-deazariboflavin ("5-deazaflavin"), a constituent of coenzyme F420 in methanogenic bacteria will be investigated in Methanobacterium thermoautotrophicum and in Streptomyces aureofaciens.